Added a ResourceManager for controlling the pace at which certain images are loaded

[hhyyrylainen]

Brief Description of What This PR Does

Used for the load game menu and the art gallery. My hope is to use this more in the future (probably when Godot 4.0 is released as background loading is pretty limited right now).

Related Issues

closes #3464
closes #3585

Progress Checklist

Note: before starting this checklist the PR should be marked as non-draft.

• PR author has checked that this PR works as intended and doesn't
  break existing features:
  https://wiki.revolutionarygamesstudio.com/wiki/Testing_Checklist
  (this is important as to not waste the time of Thrive team
  members reviewing this PR)
• Initial code review passed (this and further items should not be checked by the PR author)
• Functionality is confirmed working by another person (see above checklist link)
• Final code review is passed and code conforms to the
  styleguide.

Before merging all CI jobs should finish on this PR without errors, if
there are automatically detected style issues they should be fixed by
the PR author. Merging must follow our
styleguide.